The role of the low pressure compressor and the high pressure compressor in a turbojet is to draw air in and compress to bring it to the optimum velocity, pressure and temperature at the inlet to the combustion the chamber. The high and low pressure compressors are designed in the same manner, but there are differences in the size of their shafts and rotation speeds.
An axial compressor is composed of a set of axial stages in series, each comprising a mobile bladed wheel (or rotor) and fixed guide vanes (stator). The mobile blades of the rotor are composed of a circular disk onto which blades (rotor blades) are fixed and that rotates in front of the fixed stator vanes. Each stage of the axial compressor is sized and controlled to adapt its operating conditions perfectly to the operating conditions of other upstream and downstream stages along the direction of circulation of the air flow. In particular, the vanes of stator stages may be variable pitch, which means that it is possible to vary the angle of attack of the vanes relative to the direction of the air flow along the axis of the engine as a function of flight conditions, under the control of a slaving system.
In each stage of an axial compressor, the rotor sucks in and accelerates the air flow, deviating it relative to the axis of the engine. The next stator straightens the flow along the centre line and slows it, transforming part of its speed into pressure. In the stage directly downstream along the axis of the engine, the next rotor reaccelerates the air flow and deviates it once again from the axis of the engine, and the next stator straightens the flow once again to slow it and transform its speed into pressure. This process continues for each following stage along the axis of the engine from the upstream end to the downstream end.
In each stage, the variable pitch vanes are supported on the stator case and their position can be adjusted about the axis (or pivot) of each vane to optimise the gas flow along the axis of the engine. The angle of attack of the vanes, in other words the orientation of the variable pitch vanes of the stator relative to the axis of the engine, is controlled through one or several control elements in the form of a ring or ring segment. These ring elements or control rings are external to the case and are connected to the vanes by corresponding rods. Each of the control rings causes a change in the orientation of a plurality of vanes simultaneously.
Bushes are provided in holes formed on the inside of the control rings to hold the pins of a plurality of control levers each controlling the orientation of one vane. For example, these bushes may be made of a composite material or a metallic material. Their purpose is to assure good contact between the pin of each lever and each corresponding hole in the ring, so as to reduce friction and clearance between parts, particularly to prevent them from being deteriorated. They thus help to maintain precision of the link between the levers and the ring in the long term, since the compressor performances depend on this link.
However, a problem can arise because these bushes can break during use. Contacts between the bush and the edge at the inlet to the hole in the ring in which the lever pin fits generate a high local stress peak that can cause failure of the bush. Making a chamfer at the hole inlet can reduce this risk of failure, but cannot completely eliminate it.
The consequences of failure of a bush can vary from a reduction in the precision of the vane pitch setting, to loss of the lower part (the shank) of the bush in the core compartment of the engine and then on the tarmac when the engine covers are removed, for example for an inspection. The hole in the ring in which the control lever pin fits passes through the ring and therefore does not retain the bottom part of a bush if the bush breaks into two parts.
These effects can have serious or potentially critical consequences. Not only the increase in clearance affects good operability of the compressor, but the presence of bush shanks in the engine and then on the tarmac is not allowed according to the aircraft maintenance manual (AMM). Therefore if they are discovered during a visual inspection, airlines are obliged to make an unprogrammed engine removal to replace the broken bushes.
Finally, beyond the technical consequences on the operation of variable pitch vanes, a breakage of the bushes can have consequences on commercial operation of the plane. These consequences include particularly flight delays due to unplanned maintenance operations, leading to additional costs for airlines.
One solution to this problem could be to modify the design of the rings so as to provide means of retaining the shanks in the holes in the rings to prevent the shanks from coming free if the bushes break. This would make it possible to maintain sufficient control of the system pitch so that the compressors remain operable to a certain degree.
However, there are several disadvantages to such a “redesign” of the rings: firstly, airlines would be obliged to purchase new ring models; secondly, they would have to remove engines to be able to replace existing rings with new rings; and thirdly and finally, this “redesign” might be difficult or even impossible, at least for the IGV (Inlet Guide Vane) stages, due to the shortage of space at this ring which makes it impossible to make a new ring model larger than existing rings.